Rename GTK+ to GTK

[mortodinoite]

Hi,

I would like to suggest replacing GTK+ with "GTK" here: [1] [2] [3]

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/README.rst#n168
[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/translations/zh_CN/admin-guide/README.rst#n149
[3] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/translations/zh_TW/admin-guide/README.rst

Upstream calls it "GTK" nowadays: 
https://mail.gnome.org/archives/gtk-devel-list/2019-February/msg00000.html